Seat belt buckles in general must meet many requirements to reliably operate under any and all conditions. One specific requirement for seat belt buckles is to function when used with seat belt pretensioners (i.e., retractor, buckle or anchor pretensioners).
Seat belt pretensioners remove seat belt slack in the event of a predetermined occurrence. When pretensioners are activated this results in a very high acceleration of the seat belt webbing and subsequently the seat belt buckle.
Therefore, it is desirable to provide the seat belt buckle with a locking feature or device that is engaged during activation of the pretensioners.